(ITS#5834) enable modrDN rewriting

Full_Name: Kostantinos Koukopoulos
Version: HEAD
OS: Solaris
URL: ftp://ftp.openldap.org/incoming/Kostantinos-Koukopoulos-081128.patch
Submission from: (NULL) (195.134.100.30)
The 'modrDN' rewrite context, as mentioned in the slapo-rwm documentation, is
not implemented. Instead there is a undocumented context called 'renameDN' which
when undefined will use the default context. The submitted patch simply enables
the 'modrDN' context.